A hospital in Norfolk has called in outside experts after what it described as a significant rise in the number of stillbirths.

The Norfolk and Norwich University Hospital has asked the Royal College of Obstetricians and Gynaecologists (RCOG) to carry out an external review.

The BBC understands there have been 35 cases so far in 2011 compared with 21 in 2010.

The hospital says it has investigated each case and none was preventable.

One case is still being scrutinised.

A report to the Trust Board describes how the rise in stillbirths became apparent during the summer.

The report says an "urgent" external review has been initiated because of the "lack of evidence as to why this is happening".

The hospital's Chief Executive Anna Dugdale said: "I don't believe at the moment that there is any cause for concern but we have invited in external reviewers to give us a national context and a national perspective just to reassure both ourselves and our public."

She also pointed out that no link had been established between the cases.

"There is no discernible pattern and that's really important," she said.

The RCOG team is expected to start work in January and will report later in the year.

Stillbirth is defined as the loss of a baby after 24 weeks and there were 4,100 such cases in the UK in 2009 with a rate of 3.5 per thousand births.

A number of studies released earlier this year suggested the rate in the UK is higher than in virtually every other high income country.

According to Ms Dugdale, the Norfolk and Norwich University Hospital dealt with around 6000 births last year.